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As New Zealand tried to come to terms with shootings at two Christchurch mosques, which police said left 49 dead and dozens injured, witnesses recounted what they saw as a gunman opened fire on worshippers.

"People started to pass by us with blood stains on their clothes, they were very scared and their voices were trembling, some people could not even speak," restaurant manager Prakash Sapkota, who was near the mosques, told Euronews.

"Everything happened about 200 metres away from us, we were in the restaurant at about two in the afternoon and we heard shouting with shaking voices and rumblings like tires exploding ... We got scared, we asked ourselves what was happening," Sapkota said.

Video footage widely circulated on social media, apparently taken by the gunman, showed him driving to one mosque entering it and shooting at people at random.

Mohan Ilbrahim, a worshipper who witnessed one of the attacks inside a mosque told NBC's Today Show when he first heard the sound of the shots he thought it was "an electric short circuit" but soon realised it was a shooting after the windows began smashing.

Ibrahim said he managed to escape via the backdoor of the mosque: "Lots of people we were inside and there was a door on the right side for the ladies entrance section. And some of us we were able to come out of that door on the back side of the mosque, and there's lots of car parking (sic)."

Police Commissioner Mike Bush said 49 people were killed in total and health authorities said 48 people were being treated for gunshot wounds, including young children.

New Zealand's Prime Minister Jacinda Ardern called the shooting a "terrorist attack" and one of the country's darkest days.

The group "Muslims in New Zealand" said it was "overwhelmed" with messages of support in a Tweet.